WHETHER it's eczema or prickly heat, most rashes are usually relatively harmless.
But some marks on the skin can be deadly if left untreated, so it's important to be able to tell the difference.
Dr Walayat Hussain, a spokesman for the British Association of Dermatologists, said: "The skin is like the window to the body if you like, so sometimes that can be a sign that something is going on inside the body."
Millions of Brits suffer with common conditions like eczema, psoriasis, and acne.
Thousands more also experience hives, blisters and heat rash (also known as prickly heat).
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All of these things can be incredibly frustrating to live with - but generally, they don't pose a serious risk to your health.
However, some serious rashes have very similar symptoms, experts warn.
Read on to find out what your rash symptoms could be really caused by - and check if you'd know if you were at risk of a killer condition.
1. A patchy red rash that can turn scaly: Lymphoma
Patients diagnosed with lymphoma - cancer of the lymphatic system - sometimes experience mycosis fungoides.
The rash can be itchy and looks very similar to other common skin conditions like eczema or psoriasis, so getting a diagnosis isn't always straightforward.
Many cases develop very slowly, some taking up to 40 years.
This kind of delay can affect your treatment and prognosis.
The lymphatic system is the network of organs, vessels, tissues and glands in the body that work to get rid of toxins.
Cancer here can be particularly dangerous because the cells can travel through the body in the bloodstream.
They can then grow in several places at once, affecting multiple parts of the body.
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A person with lymphoma may develop mycosis fungoides when the blood travels to the upper layer of the skin.
In its early stages, it may appear as a patchy red rash that is raised and chronically itchy.
But as it progresses, it tends to become scaly, with thin, wrinkled skin.
About 450 people are diagnosed with mycosis fungoides each year in the UK, according to the British Association of Dermatologists.
There is no cure, and even after treatment, the condition usually comes back.
Almost 90 in 100 people survive five years or more after diagnosis, Cancer Research UK says.
2. Tiny red spots covering the skin: Leukaemia
Another type of cancer that can cause a rash us leukaemia.
It is a type of blood cancer that starts in the bone marrow and can spread throughout the body in the bloodstream.
As well as fatigue and bruising, symptoms can include petechiae - clusters of tiny spots on their skin.
On lighter skin, these usually look red, and on darker skin, they can appear purple or darker than the surrounding skin.
The rash is caused by broken blood vessels under the skin and often resembles atopic dermatitis - one of the most common forms of eczema.
The main difference is that leukaemia rashes will retain their red, purple or brown colour when pressure is applied, while most other rashes will turn white.
3. Red spots UNDER the skin: Blood clotting issues
If you notice purple spots under the skin, this could signal a blood clotting disorder.
These dots can be tiny pin prick-sized or large patches.
The rash, called purpura, can be caused by low or high levels of platelets - cells in the blood that cause clotting.
Weak blood vessels, scurvy, medications or vaccines, infection such HIV or Hepatitis C, and conditions present from birth can all cause purpura.
Blood clotting disorders can be incredibly dangerous.
For example, Thrombotic Thrombocytopenia Purpura (TTP) causes extensive clots to form in small blood vessels throughout the body and is considered a true life-threatening medical emergency, according to the NHS.
4. Reddish rash on your hands or back: Cancer
Dermatomyositis is another skin trait to watch out for.
It tends to appear as changes around your nail folds, a reddish rash on the back of your hands, especially around the knuckles, and a shawl-like red rash on your back.
It can also appear on your face, eyelids, chest, knees and elbows.
As well as skin alterations, dermatomyositis can cause muscle weakness and inflamed muscles.
It is associated with many types of cancer.
While myositis doesn’t cause cancer, researchers have found that a higher than normal percentage of patients with it also develop cancer.
This includes ovarian, lung, pancreatic, stomach, lymphatic, bowel and breast.
5. Red pin pricks with flu-like symptoms: Meningitis
A red pin prick rash under the skin can indicate meningitis - a sometimes fatal condition involving inflammation of the lining around the brain and spinal cord.
The symptoms of meningitis mimic flu (high fever, stiff neck, headache and sleepiness), and the rash may not always occur early on.
It starts as small, red pin pricks before spreading quickly and turning into red or purple blotches, the NHS says.
It can be harder to see on darker skin, so check paler areas, such as the palms of the hands, soles of the feet, roof of the mouth, tummy, whites of the eyes and the inside of the eyelids.
You can check if you or your child has it by pressing a glass against the rash - if it does not fade under pressure, then it is caused by meningitis.
It can be a sign of sepsis caused by meningitis and you should call 999 straight away.
All parents are told to vaccinate their babies against meningitis very early on.
The charity Meningitis Now estimates around 8,000 people get meningitis every year in the UK.
Some 2.5million causes are recorded globally, with around 250,000 deaths, according to non-profit PATH.
6. Dark, thicker patches of skin: Cancer or diabetes
Acanthosis nigricans is when darker, thickened patches of skin develop around the armpit, groin or neck.
It's not a condition in itself but a sign of an underlying health problem.
The patches are usually dry and rough, feeling similar to velvet, and can also be itchy.
Some people also have tiny growths (skin tags) on the patches.
The most common cause of acanthosis nigricans is being very overweight, the NHS says.
Other potential causes include type 2 diabetes, Cushing's syndrome. polycystic ovary syndrome (PCOS), an underactive thyroid, steroids, hormone treatments, the contraceptive pill, stomach cancer and a faulty gene inherited from your parents.
While some of these conditions themselves aren't necessarily deadly, they have a number of potentially serious complications.
For example, people with diabetes are at increased risk of high blood pressure, heart attacks and amputations, so it is vital to get an early diagnosis.
Those with Cushing's syndrome can also experience high blood pressure, as well as bone loss, strokes, infections and blood clots.
Sometimes healthy people with no other conditions get acanthosis nigricans. This is more common in people with black or brown skin.
7. Purplish rash with swelling, blackening: Rotting flesh
An immediate medical concern, necrotising fasciitis is a rare but serious bacterial infection that causes the skin to “rot”.
Necrotising fasciitis, also known as the "flesh-eating disease", can start from a relatively minor injury, such as a small cut, which allows bacteria to get into the skin.
Symptoms usually come on suddenly within hours or over a few days.
At first you might have intense pain or loss of feeling near a cut or wound.
The affected area may then swell and show a purplish rash.
This leads to large dark marks forming that turn into pus-filled blisters. The wounds blacken as the tissue dies.
You might also experience flu-like symptoms, such as a high temperature, headache and tiredness.
Just four or five days after the infection starts, a person can become seriously ill, with dangerously low blood pressure.
Even with treatment, it's estimated that up to one in every five cases of necrotising fasciitis end in death, the Centers for Disease Control and Prevention warns.
8. Widespread rash: Drug allergy
A rash that covers a wide area could be a sign of drug hypersensitivity syndrome (DRESS) - when a medication causes an allergy.
DRESS can take a few weeks to appear after you start a new medication, and continue for a while even if you stop taking it.
Patients report a diverse range of rashes - from blistering and lesions to eczema and facial swelling.
DRESS can lead to issues with internal organs, which is why it's so serious.
It could cause multi organ failure, acute liver failure, or lead to lung disease, inflammation of the heart and seizures.
DRESS is relatively rare, and affects men and women equally, according to DermNet.
The most common drugs to cause this reaction are a number of anticonvulsant drugs (particularly carbamazepine, phenobarbital, and phenytoin), the anti-gout drug, allopurinol, olanzapine, and the sulphonamide group of antibiotics.
It has been estimated that at least one in every 10,000 patients treated with an anticonvulsant will develop DRESS.
BUT... in most cases a rash is harmless
With skin rashes linked to so many serious diseases, including cancer, it can be easy to panic.
But Dr Hussain reassured that most of the time, a rash is harmless.
"The most common cause of a rash will not be cancer related," he said.
"I think that is a really important message to give to people.
"The bottom line is, if there is anything on your skin that you are worried about,you go and see your GP."
